Aquatic animal showing mass discharge (7)
I believe the answer is:
grampus
'aquatic animal' is the definition.
(term for a dolphin or whale)
'mass discharge' is the wordplay.
'mass' becomes 'gram' (unit of mass).
'discharge' becomes 'pus' (discharge from the body).
'gram'+'pus'='GRAMPUS'
'showing' is the link.
(Other definitions for grampus that I've seen before include "Popular name for a killer whale" , "Dolphin - killer whale" , "Killer-whale" , "huge creature" , "Killler-whale" .)